Graphene and boron-nitride films are examples of useful large area thin films that may be beneficially produced using the methods and apparatus of the present invention. The invention is particularly useful in the synthesis of graphene, which is a one-atom thick, two-dimensional planar sheet or monolayer in which carbon atoms are bonded in a stable extended fused array, comprising polycyclic aromatic rings with covalently bonded carbon atoms having sp2 orbital hybridization. The covalently bonded carbon atoms are densely packed in a honeycomb crystal lattice, and may form a 6-membered ring as the basic repeating unit, but 5-membered rings and/or 7-membered rings may also be formed. Graphene has distinctive electrical, mechanical and chemical properties that make it attractive for applications in flexible electronics. For example, electrons may move on a graphene sheet as though they have zero mass, and thus may move at the velocity of light.
Graphene may be formed on the surface of a substrate by a variety of methods. An exemplary but promising method is set forth in U.S. Patent Application Publication No. 2011/0091647 (the disclosure of which is incorporated by reference herein in its entirety), in which graphene may be produced using a chemical vapor deposition (CVD) process. In general, CVD is a process by which a thin film layer is deposited onto a substrate. The substrate is supported in a vacuum deposition process chamber, and the substrate is heated to a high temperature, typically several hundred degrees Celsius. Deposition gases are then injected into the chamber, and are thermally activated such that a chemical reaction takes place by which a thin film layer is deposited onto the substrate. The substrate on which the thin film layer has been deposited is then cooled to room temperature, after which the thin film layer may be separated from the substrate.
The aforementioned U.S. Patent Application Publication No. 2011/0091647 discloses a CVD process by which graphene may be formed on a metallic substrate such as copper foil, which is heat-treated in the presence of a gaseous carbon source, specifically, a mixture of a hydrocarbon gas and hydrogen gas. The metallic substrate is loaded into a tube furnace, usually comprising a cavity where the heat treatment is carried out at a specified temperature; the cavity is generally surrounded by heating elements and is in fluid communication with the gaseous sources. It is believed that during the treatment, some of the heated hydrogen gas disassociates into atomic hydrogen, which then reacts with the hydrocarbon gas, typically methane, to form one or more carbon growth species that, when they come into contact with the slightly cooler metallic substrate, form a deposit on the substrate as an thin carbon film (graphene). After a specified period of time the heat treatment is terminated, and the furnace, along with the coated substrate, is then cooled to room temperature, after which the thin film may used directly, with the substrate still attached, or may be separated or transferred from the substrate in a known manner, and then used.
In a similar manner, thin films of boron-nitride can be obtained by CVD, from precursors such as boron trichloride or boron tribromide and either nitrogen or a source of nitrogen such as ammonia, using a metallic foil substrate. Moreover, in addition to utilizing conventional CVD, graphene films, boron-nitride films and other large area thin films can alternatively be produced using other related processes, such as plasma-enhanced CVD (PECVD), as well as by using similar processes such as atomic layered deposition (ALD).
In each of these production techniques, the size (i.e., the surface area) of the thin film that is produced is determined by the size of the substrate on which it is grown, and the latter is limited, in turn, only by the dimensions of the reactor chamber or cavity of the CVD apparatus that is used. In general, that chamber is a horizontally-oriented cylindrical quartz tube, and the dimensions of the substrate which may be used are circumscribed by the dimensions of the chamber—the length of the substrate, which is defined as the dimension parallel to the axis of the cylindrical chamber, is determined by the length of that portion of the chamber which can be heated (that is, by the length of the heating zone of the furnace), and the width of the substrate, which is defined as the dimension perpendicular to the length and which is parallel to a radial dimension of the cylindrical chamber, is determined by the diameter of the chamber (that diameter being about equal to the maximum substrate width that can be obtained when the conventional technique of placing a flat sheet of the substrate into the cylindrical chamber is utilized). Although there are relatively few technical difficulties involved in manufacturing very long quartz tubes, and in manufacturing tube furnaces that would accommodate such tubes, the difficulty in manufacturing larger diameter quartz tubes increases dramatically with the increase in diameter of the tube. In addition, the connections between the quartz tube and the surrounding metal parts become more problematic with increased diameters because the manufacturing errors in the size and/or roundness of the quartz tube become magnified. Therefore, there are practical limits as to the diameter of the quartz tube reactor chamber, which in turn limit the width of the substrate that may be utilized therein.
Accordingly, a technique by which to load a metallic foil substrate into the quartz tube reactor chamber, such that the width of the substrate, and therefore the width of the thin film subsequently produced, can be increased dramatically despite the limits imposed by the diameters of presently-existing reactor chambers for CVD furnaces, would be a useful addition to the technology by which such films are manufactured using CVD or similar processes. Yet, despite the existence and availability of CVD processes for many years, such a technique has eluded researchers.
Although efforts have been made in the prior art to provide such techniques, those efforts are not completely satisfactory. For example, the prior art includes a technique by which the substrate can be wrapped around a cylindrical holder, but this provides an increase in substrate width of only about three times that of the chamber diameter itself. Moreover, although the aforementioned U.S. Patent Application Publication No. 2011/0091647 appears to disclose that a copper foil substrate about 2 meters in width may be utilized in the production of graphene, thus implying that the graphene film produced could have a comparable width, it has been determined that a flat substrate of such a width dimension would not be workable as a practical matter, due to the manufacturing and other difficulties mentioned above that are associated with producing quartz tube reaction chambers of increased diameter.
It is therefore the principal object of the present invention to provide improved methods and apparatus for synthesizing large area thin films using a CVD or CVD-type furnace, in which the width dimension of the thin film produced may be greatly increased.
It is another object of the present invention to provide improved methods of supporting a substrate, and an improved substrate support apparatus for heating a substrate, in the reactor chamber of a CVD or CVD-type furnace.